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cmd/compile: some unreasonable in BCE decision making for global slices #47736

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go101 opened this issue Aug 17, 2021 · 2 comments
Closed

cmd/compile: some unreasonable in BCE decision making for global slices #47736

go101 opened this issue Aug 17, 2021 · 2 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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@go101
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go101 commented Aug 17, 2021

What version of Go are you using (go version)?

$ go version
go version go1.17 linux/amd64

Does this issue reproduce with the latest release?

Yes

What did you do?

package main

var s = make([]int, 100)

func f() (r int) {
	for i := range s {
		r += s[i] // bound check eliminated
	}
	return r
}

func g() {
	for i := range s {
		s[i] = i // need bound check
	}
}

func main() {
}

What did you expect to see?

Both bound checks are eliminated or both not.

What did you see instead?

One is eliminated but the other is not.
@go101 go101 mentioned this issue Aug 17, 2021
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@mknyszek mknyszek added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Aug 17, 2021
@mknyszek mknyszek added this to the Backlog milestone Aug 17, 2021
@mknyszek
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CC @randall77 @mdempsky via https://dev.golang.org/owners

@randall77
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I believe this is because there are no writes in f. All the loads of s get coalesced into a single load, so prove can tell that the slice you're ranging over is the same as the one you're indexing.
In g, there's a write, so the s inside the loop is reloaded every time. That s might have a different length, so a bounds check is needed.

I don't think this qualifies as a bug. Closing. Maybe there's something that could be done with g, like proving that the location written doesn't alias the s global, but we haven't gone there yet.

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